In general, a fixing device to be installed in an image forming apparatus such as an electrophotographing system copying machine, printer, or the like, is configured to heat a recording material where an unfixed toner image is carried to fix the toner image on the recording material while transporting the recording material by a nip portion formed of a heating rotary member and a pressure roller which is in contact therewith.
In recent years, an electromagnetic induction heating system fixing device whereby an electroconductive layer of a heating rotary member can directly be heated has been developed and put into practice. The electromagnetic induction heating system fixing device has an advantage in that warm-up time is short.
With fixing devices disclosed in PTL 1, PTL 2, and PTL 3, according to an eddy current induced in an electroconductive layer of a heating rotary member with a magnetic field generated from a magnetic field generator, the electroconductive layer is heated. With such fixing devices, as the electroconductive layer of the heating rotary member, magnetic metal which readily passes magnetic flux such as iron or nickel or the like of which the thickness is 200 μm to 1 mm, or an alloy primarily made up of these, is employed.
Incidentally, in order to attempt to reduce warm-up time of a fixing device, heat capacity of the heating rotary member has to be reduced, and accordingly, it is advantageous that the thickness of the electroconductive layer of the heating rotary member be small. However, with the fixing devices disclosed in the above-mentioned literatures, reducing the thickness of the heating rotary member being reduced, results in deterioration of heat efficiency. Further, with regard to the fixing devices disclosed in the above-mentioned literatures, even in the event of employing a material of which the relative permeability is low, heat efficiency deteriorates. Therefore, with the fixing devices disclosed in the above-mentioned literatures, a thick material having high relative permeability has to be selected as the material of the heating rotary member.
Accordingly, the fixing devices disclosed in the above-mentioned literatures have a problem in that a material to be used as the electroconductive layer of the heating rotary member is restricted to a material having high relative permeability, and restraints are imposed on costs, material processing method, and device configuration.